Bridges seem simple: just metal structures that let us cross over rivers, roads, or valleys. But behind every bridge is careful design and engineering. So how do bridges actually stay up even under the weight of cars, trucks, and time? Let’s break it down.

The Forces at Play

All bridges have to deal with a few key forces:

• Compression – a force that pushes materials together (like squeezing).
• Tension – a force that pulls materials apart (like stretching).
• Load – the weight the bridge supports, including its own weight and anything moving on it (usually people, cars, trains, etc)
• Shear and torsion – twisting and sliding forces bridges must resist (usually from the wind).

Different types of bridges balance these forces in different ways.

Different Types of Bridges

Beam Bridges
The simplest kind. A flat beam supported at both ends. Most highway bridges are beam bridges. The top of the beam is in compression, and the bottom is in tension. They’re best for short spans.

Arch Bridges
These use curved arches to transfer weight. The arch pushes the force outward and downward into what are called abutments (supports at the ends of a bridge). The shape naturally handles compression very well, which is why arch bridges can be made of stone or concrete.

Suspension Bridges
Think of the Golden Gate Bridge. These use tall towers and cables. The main cables hang between the towers and hold the vertical cables, which support the road. The towers take on the compression, while the cables handle the tension. This design allows for very long spans.

Cable-Stayed Bridges
Similar to suspension bridges, but here the cables connect directly from the tower to the roadway in straight lines. They’re modern, efficient, and often seen in city skylines.

Truss Bridges
These use a triangle framework (called a truss) made of beams. Triangles are strong shapes that distribute weight efficiently, making truss bridges sturdy and good for medium spans.

Want to Learn More?

PBS NOVA – “Build a Bridge” Interactive
pbs.org/nova/bridge
Try building your own bridge and see what holds and what collapses.

YouTube: “How Do Bridges Work?” by Practical Engineering
Watch here
More in-depth explanation of bridge design and the forces they are built to handle.